The invention generally relates to transformer systems, and relates in particular to transformer hybrid systems for use with modem systems.
Many modem systems include a hybrid matching network to facilitate the transmission of a signal, to permit reception of a signal with reduced attenuation, and to reduce interference from the signal transmission path into the signal reception path. The hybrid matching network provides the interface between the modem circuit and the transmission line, e.g., the tip and ring of the telephone network. The hybrid must provide the proper termination, isolation, signal amplification and hybrid rejection required for modem operation.
In particular, many conventional hybrid networks employ a transformer to provide the required isolation barriers between sensitive electronic circuitry and the telephone line. This transformer is also used to step the transmit (TX) voltage up or down depending on the application. The hybrid network may also properly accept the receive (RX) signal while keeping the TX signal from entering the RX path and contaminating the RX signal (e.g., with an echo signal).
As shown in FIG. 1, a conventional hybrid matching network may include a pair of transmit differential signal nodes 10, 12, a pair of receive differential signal nodes 14, 16, and transmit line differential signal nodes 18, 20 for coupling to the tip and ring of a telephone network. The circuit also includes a balanced Wheatstone bridge including windings (N1) 22, 24 on the integrated circuit side and impedances (Zm) 26, 28 and further includes windings (N2) 30, 32 on the line side. Ideally, impedances (Zm) 26, 28 are chosen to be identical to the reflected line impedances as seen through the N1 windings. Half of the TX signal, therefore, is dropped across each of the N1 windings. The total voltage across the two N2 windings is the TX signal multiplied by the turns ratio N2/N1. The turns ratio may, therefore, be used to control any stepping up or down of the TX signal.
The transmit differential signal nodes 10, 12 should effectively appear to be ground to the RX signal. Each of the N1 windings sees half of the RX signal reflected by the turns ratio. The complete reflected RX signal, therefore, appears at the receive differential signal nodes 14, 16.
The interference rejection (echo rejection) is achieved by employing a balanced bridge such that no component of the TX signal appears at the receive differential signal nodes 14, 16. The closer that the impedance Zm is matched to the line impedance reflected through the N1 windings, the better the circuit will provide echo rejection. The telephone line may typically be modeled with an RC circuit, although it is sometimes helpful to also include an inductor in Zm to match the effect of the transformer inductance. The use of inductors in the matching network, however, is not generally desired due to their size, cost and/or noise sensitivity. The matching impedance Zm, therefore, is typically implemented using only resistors and capacitors, and the hybrid matching is typically optimized for a specific desired frequency range.
In certain applications, the swing range of the TX signal may not be large enough to provide the desired voltage to nodes 18 and 20. If a transformer is used to step the TX voltage up on the line side, then the value of the capacitors in Zm may become too large and/or expensive. For example, a three-fold increase in the turns ratio (e.g., from 1:1 to 1:3) may require a nine fold increase in the size of the required capacitance in Zm.
There is a need, therefore, for more efficient and cost effective implementation of a hybrid matching network.